Field of the Invention
The present invention relates to an automatic transmission for a vehicle, and more particularly, to a planetary gear train of an automatic transmission for a vehicle which implements 9 forward speeds and increases an entire gear ratio by a minimum configuration to improve power transmission performance and fuel efficiency and secure linearity of step ratios between transmission stages.
Description of Related Art
In recent years, a rise in oil price causes dashing into unlimited competition for enhancing fuel efficiency.
As a result, researches into reduction of a weight and the enhancement of the fuel efficiency through downsizing are conducted in the case of an engine and researches for simultaneously securing operability and fuel efficiency competitiveness through multistages are conducted in the case of an automatic transmission.
However, in the automatic transmission, as transmission steps increase, the number of internal components increases, and as a result, mountability, cost, weight, transmission efficiency, and the like may still deteriorate.
Accordingly, development of a planetary gear train which may bring about maximum efficiency with a small number of components may be important in order to increase a fuel efficiency enhancement effect through the multistages.
In this aspect, in recent years, 8 and 9-speed automated transmissions tend to be implemented and the research and development of a planetary gear train capable of implementing more transmission steps has also been actively conducted.
However, improvement effect of fuel efficiency is not so good in case of 8-speed automated transmission, because an entire gear ratio is maintained in a level of 6.5˜7.5.
Therefore, development of a high efficiency automatic transmission implementing at least 9 speeds is required in that operation efficiency of an engine and drivability of a vehicle are deteriorated because linearity of step ratios between transmission stages can't be secured in case of enabling an entire gear ratio of an 8-speed automated transmission to be a level more than or equal to 9.0.
The information disclosed in this Background of the Invention section is only for enhancement of understanding of the general background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art already known to a person skilled in the art.